1. Technical Field of the Invention
The present invention relates in general to multiplexing users in a telecommunication system, and in particular to a method and system for accessing communication resources in a radio network.
2. Description of Related Art
With the introduction of new services or applications over packet data systems in a mobile communication network, for example, real time (RT) services such as VoIP, there will be a large variety of quality of service (QoS) demands on the network. Certain users, for example, those utilizing real time voice applications will have a very high demand for the availability of transmission resources, whereas users, for example, who transmit short messages or electronic mail, will be satisfied with a lower availability of transmission resources.
For example, in the well known Universal Mobile Telecommunications System (UMTS), there are four proposed QoS classes: the conversational class; streaming class; interactive class; and background class. The main distinguishing factor between these classes is the sensitivity to traffic delay. Conversational class traffic is intended for traffic which is very delay sensitive while background class traffic is the most delay insensitive traffic class. Conversational and streaming classes are intended to be used to carry RT traffic flows and interactive and background classes are intended to be used to carry Internet applications (e.g., www, e-mail, telnet, FTP, etc.).
Real time services include sensitive time constraints over a reserved access channel. That is, delays in the transmission and/or reception of successive packets can have noticeable and undesirable QoS effects (e.g., on voice quality). These time constraints can be handled by always reserving access time at predetermined intervals during a communication with high QoS demands. In this way, a real time service communication can proceed uninterrupted since it will be allocated communication resources regardless of whether or not any packets will be sent. That is, for example, silent periods will occur in a real time voice communication, and in order to conserve battery resources, the silent periods need not be transmitted, or the transmission power can be considerably reduced.
Silent periods can be detected in a voice activity detector (VAD) device. During silent periods, a silence descriptor (SID) signal is sent to the receiver. The receiver generates comfort noise in order to closely mimic the naturally occurring background noise so that the receiving user perceives that the communication path between the transmitter and the receiver is still open and operable. In addition to the SID, an indication is sent to the transmitter that there is no voice activity detected and the transmitter can reduce its transmitter output power or set it to zero for that connection. This technique is called discontinuous transmission (DTX). With DTX enabled, interference is decreased in the system, since transmitters will only emit output power when there is information to be transmitted (e.g., when voice activity is detected or when SIDs are transmitted).
Resources are allocated for the real time services users regardless of whether or not packets are sent from the transmitter. However, it would be advantageous if these silent periods could be used in a more efficient way by allowing other applications to use the allocated resources during the silent periods without lowering the QoS of real time service.
In connection with the development of third generation mobile communication systems, new wireless multimedia and data applications are being designed and introduced. To support these new applications, improved data transmission technologies are also being developed. One such technology is Enhanced Data rates for Global Evolution (EDGE), which uses a more efficient radio-modulation technology that is optimized for data communications and that can be implemented on existing GSM and IS-136 systems. When used in connection with General Packet Radio Service (GPRS), a packet-switched technology that delivers speeds of up to 115 kilobits per second (kbit/s), EDGE technology can increase end user data rates up to 384 kbit/s, and potentially higher in high quality radio environments.
In connection with the development of EDGE and other technologies for supporting higher data rates, a number of techniques for multiplexing different users on the same set of resources have been developed. For example, in the packet-switched mode of EDGE technology (i.e., Enhanced GPRS (EGPRS)), the capability exists to multiplex different users on the same time slot. In this mode, packet data is transferred via a wireless communication link using 20 millisecond (ms) radio blocks. Each radio block is transferred to or from a particular user as a sequence of four consecutive bursts on a time slot that is assigned to the user. Subsequently, the time slot can be assigned to another user for the transmission of four bursts to or from that other user or can be again assigned to the same user for the transmission of an additional four bursts.
To send data on the downlink, the base station knows when new packets need to be transmitted to each user. Accordingly, the base station transmits data destined for a particular user as part of a temporary block flow (TBF). The TBF is a connection used by the base station and the user's mobile station to support the unidirectional transfer of packet data on a packet data physical channel. The network assigns each TBF a temporary flow identity (TFI) value, which uniquely identifies the TBF, thereby distinguishing the TBF from other TBFs destined for other mobile stations. Based on the TFI value, each individual mobile station that is multiplexed on a specific packet data physical channel is able to determine which data packets are meant for that mobile station. In other words, the base station is able to address data packets to particular mobile stations using the appropriate TFI value. On the uplink portion of the communication, however, the situation is more cumbersome because the base station does not know which mobile stations need to send data packets unless and until the mobile stations notify the base station that they have data to be sent.
To facilitate data transfers on the uplink, therefore, a mobile station that needs uplink resources informs the base station that it has data packets to send by sending a message on the random access channel (RACH) or the packet RACH (PRACH), which are control channels used only on the uplink to request GPRS resources. The base station can then schedule uplink resources for the mobile station by sending an uplink state flag (USF) in the header of a radio block that is sent on the downlink. The USF serves to identify the specific mobile station that is authorized to send data packets in the next uplink radio block.
The problem with this process is that the radio channel activation procedure (i.e., for a mobile station to obtain authorization to use a radio channel on the uplink) can take a relatively long time (up to 300 ms), even if the mobile station successfully obtains authorization on the first attempt to request such access. In some cases, however, a collision can occur between two or more different mobile stations attempting to request access to the radio channel at the same time. When such a collision occurs, the mobile station backs off and waits until a later time to resend the request. As a result, the delay for accessing the radio channel is further increased.
In connection with more recent EGPRS standards (i.e., EGPRS standard release 00), real time applications (e.g., packet data speech) will be supported. In addition, it will be possible to multiplex real time users with non-real time users on the same time slot. This can be accomplished by transmitting the non-real time users' blocks during silent periods of the real time user, such as between the talkspurts of a speech user. The delay inherent in existing radio channel activation procedures, however, is generally unacceptable for real time applications, particularly in the case of speech applications because the first blocks of a talkspurt are very important to maintain users' perceptions of high speech quality. Accordingly, real time users must be able to access the radio channel much faster than is supported by existing procedures.
There is a need, therefore, for a method and system that would allow real time users to quickly and efficiently obtain access to uplink radio channels for purposes of transmitting packet data. The system and method should allow multiplexing of real time users with other users on the same radio channel. Preferably, requests for such access would also require only a minimal amount of bandwidth so as to avoid using up valuable radio resources.